ORGANIC LETTERS
Divergent Synthesis of Cytotoxic Styryl Lactones from D-Xylose. The First Total Synthesis of (+)-Crassalactone C
2007 Vol. 9, No. 21 4235-4238
Velimir Popsavin,*,† Goran Benedekovic´,† Bojana Srec´o,† Mirjana Popsavin,† Jovana Francuz,† Vesna Kojic´,‡ and Gordana Bogdanovic´‡ Department of Chemistry, Faculty of Sciences, UniVersity of NoVi Sad, Trg D. ObradoVic´ a 3, 21000 NoVi Sad, Serbia, and Institute of Oncology Sremska Kamenica, Institutski put 4, 21204 Sremska Kamenica, Serbia
[email protected] Received July 26, 2007
ABSTRACT
A new divergent approach to (+)-goniofufurone (1) and 7-epi-(+)-goniofufurone (2), as well as the first total synthesis of crassalactone C (3), has been achieved starting from D-xylose. In a preliminary bioassay, all three natural products 1, 2, and 3 showed remarkable in vitro antiproliferative activities against K562, Raji, and HeLa neoplastic cell lines.
Asian trees of the genus Goniothalamus of the plant family Annonaceae have long been recognized as a source of biologically active styryl lactones.1 Many styryl lactones that have been isolated from Goniothalamus species or synthesized exhibited a notable cytotoxic activity against certain human tumor cell lines.2 (+)-Goniofufurone (1) and 7-epi(+)-goniofufurone3 (2) are naturally occurring styryl lactones that have attracted considerable attention since their isolation from the stem bark of Goniothalamus giganteus (Annonaceae).4,5 Due to their unique structural features and promis†
Department of Chemistry. Institute of Oncology. (1) For a review on chemistry, biogenesis, and biological activities of styryl lactones from Goniothalamus species, see: Blazquez, M. A.; Bermejo, A.; Zafra-Polo, M. C.; Cortes, D. Phytochem. Anal. 1999, 10, 161. (2) For recent reviews on cytotoxicity of styryl lactones and their analogues, see: (a) de Fatima, A.; Modolo, L. V.; Conegero, L. S.; Pilli, R. A.; Ferreira, C. V.; Kohn, L. K.; de Carvalho, J. E. Curr. Med. Chem. 2006, 13, 3371. (b) Mereyala, H. B.; Joe, M. Curr. Med. Chem. Anti-Cancer Agents 2001, 1, 293. (c) See also ref. 1 (3) Due to differences in numbering systems, compound 2 is sometimes named as 8-epi-goniofufurone (e.g., see ref 8). (4) Fang, X. P.; Anderson, J. E.; Chang, C. J.; Fanwick, P. E.; McLaughlin, J. L. J. Chem. Soc., Perkin Trans. 1 1990, 1655. (5) Fang, X. P.; Anderson, J. E.; Chang, C. J.; McLaughlin, J. L.; Fanwick, P. E. J. Nat. Prod. 1991, 54, 1034. ‡
10.1021/ol701734s CCC: $37.00 Published on Web 09/15/2007
© 2007 American Chemical Society
ing antitumor activities, both natural products 1 and 2, along with a number of their analogues, have been the targets of many total syntheses.6,7 (+)-Crassalactone C (3) is a natural 7-O-cinnamoyl derivative of (+)-goniofufurone that was very recently isolated from the leaves and twigs of Polyalthia crassa.8 Its structure was determined on the basis of spectroscopic methods. The absolute stereochemistry of 3 was established by treatment of the isolated (+)-1 with cinnamoyl chloride. Apart from this nonselective and lowyielding single step route,8 no total synthesis of 3 was hitherto reported. As a part of our continuing interest in the synthesis of natural products and analogues having γ-lactone rings,9 we have planned the synthesis of 1-3. We report herein their (6) For recent reviews on syntheses of styryl lactones, see: (a) Mondon, M.; Gesson, J.-P. Curr. Org. Synth. 2006, 3, 41. (b) Zhao, G.; Wu, B.; Wu, X. Y.; Zhang, Y. Z. Mini-ReV. Org. Chem. 2005, 2, 333. (7) (a) Sartillo-Melendez, C.; Cruz-Gregorio, S.; Quintero, L.; SartilloPiscil, F. Lett. Org. Chem. 2006, 3, 504. (b) Mihovilovic, M. D.; Bianchi, D. A.; Rudroff, F. Chem. Commun. 2006, 3214. (c) Ferna´ndez de la Pradilla, R.; Ferna´ndez, J.; Viso, A.; Ferna´ndez, J.; Go´mez, A. Heterocycles 2006, 68, 1579. (d) Prasad, K. R.; Gholap, S. L. Synlett 2005, 2260. (e) Ruiz, P.; Murga, J.; Carda, M.; Marco, J. A. J. Org. Chem. 2005, 70, 713. (8) Tuchinda, P.; Munyoo, B.; Pohmakotr, M.; Thinapong, P.; Sophasan, S.; Santisuk, T.; Reutrakul, V. J. Nat. Prod. 2006, 69, 1728.
divergent synthesis using D-xylose as a chiral precursor and a preliminary bioassay against human K562, Raji, HeLa, and MRC-5 cell lines. The data related to cytotoxic activities of 1-3 against the mentioned cell lines are herein for the first time reported. All three target compounds 1-3 contain five contiguous stereocenters and display a clear structural similarity. We thus wanted to design a divergent synthesis for all three compounds from a common intermediate. Among other methods, the required [3.3.0] bicyclic lactone core could be formed through condensation of Meldrum’s acid with a protected sugar lactol derivative.10 Accordingly, we envisaged the retrosynthetic concept depicted in Scheme 1. For both 1
Disconnection of 3 shows that it can be derived from the diol 14 by a number of successive transformations that involve regioselective Mitsunobu reaction in the presence of cinnamic acid and hydrolytic removal of the 1,2-Ocyclohexylidene protective group, followed by γ-lactone formation. Intermediate 14 in turn should be accessible from 5 by removal of benzyl protective group from C-3.
Scheme 2.
Synthesis of (+)-Goniofufurone (1) and 7-epi-Goniofufurone (2)
Scheme 1. Retrosynthetic Analysis of (+)-Goniofufurone (1), 7-epi-Goniofufurone (2), and (+)-Crassalactone C (3)
and 2, lactone ring-removal would give rise via, respectively, 1a and 2a to the two epimeric alcohols 6 and 5. These can be prepared from the same aldehyde 4 by means of stereoselective addition of PhMgBr. Synthesis of 4 itself is visualized from D-xylose by established chemical reactions.9b (9) (a) Popsavin, V.; Srec´o, B.; Krstic´, I.; Popsavin, M.; Kojic´, V.; Bogdanovic´, G. Eur. J. Med. Chem. 2006, 41, 1217. (b) Popsavin, V.; Krstic´, I.; Popsavin, M.; Srec´o, B.; Benedekovic´, G.; Kojic´, V.; Bogdanovic´, G. Tetrahedron 2006, 62, 11044. (c) Popsavin, V.; Grabezˇ, S.; Popsavin, M.; Krstic´, I.; Bogdanovic´, G.; Divjakovic´, V. Tetrahedron Lett. 2004, 45, 9409. (d) Popsavin, V.; Krstic´, I.; Popsavin, M. Tetrahedron Lett. 2003, 44, 8897. (10) Bruns, R.; Wernicke, A.; Koll, P. Tetrahedron 1999, 55, 9793. 4236
The synthesis of 1 and 2 is presented in Scheme 2. Addition of phenylmagnesium bromide in ether to 4 led to two diastereomeric alcohols 5 and 6 in a 6:1 ratio and 82% combined yield. The observed diastereoselectivity may be explained as a result of the 1,2-chelation control.11 The major L-ido isomer 5 has the same stereochemistry as target 2, while the minor D-gluco stereoisomer 6 is of the same absolute (11) Gracza, T.; Szolcsa´nyi, P. Molecules 2000, 5, 1386.
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configuration as both targets 1 and 3. Efforts to change the isomeric ratio in favor of 6 were unsuccessful (e.g., use of phenylcerium dichloride11 gave 5 and 6 in a 2:1 ratio, but in only 20% yield). To prepare compound 6, an efficient twostep route was developed that involved configurational inversion at C-5 in 5 under the Mitsunobu conditions.12 Accordingly, treatment of 5 with diethyl azodicarboxylate, triphenylphosphine, and benzoic acid gave the expected 5-Obenzoyl derivative 7 which upon deprotection with methanolic sodium methoxide formed 6 in 55% yield (from three steps). Hydrolytic removal of the cyclohexylidene protective group in 6 with aqueous acetic acid, gave the corresponding lactol 8. Since the pyridine solution of 8 mutarotated to a more positive equilibrium value {[R]20D ) +23.7 f +39.6 (77 h)}, the crystalline 8 was the β anomer (Hadson’s rule). Attempted condensation of 8 with Meldrum’s acid in the presence of tert-butylamine10 failed to give the expected γ-lactone 9. However, when the last reaction was carried out in the presence of triethylamine as a catalyst,13 the desired γ-lactone 9 was obtained in 52% yield. Final cleavage of benzyl protecting group in 9 gave (+)-goniofufurone (1). The physical and spectral data of thus-obtained sample 1 were identical to those reported in the literature.7e,10 The stereoisomer 5 was converted to (+)-7-epi-goniofufurone (2) by using the same methodology as that already applied for the conversion of 6 to 1. Treatment of 5 with aqueous acetic acid gave 10 (64%) along with a minor amount of 11 (9%).14 Reaction of 10 with Meldrum’s acid, followed by hydrogenolytic 5-O-deprotection gave (+)-7epi-goniofufurone (2). All physical constants and spectral data of thus prepared natural product 2 were in good agreement with those reported in the literature.10,15 The synthesis of (+)-crassalactone C (3) is presented in Scheme 3. Removal of the benzyl protective group in 5 gave the corresponding diol 13 (70%) that was further allowed to react with cinnamic acid under the standard Mitsunobu conditions. The corresponding cinnamic ester 14 was thus obtained (63%) accompanied by a minor amount of 3,5anhydro derivative 15 (16%). The side product 15 was presumably formed by a competitive intramolecular nucleophilic displacement process.12 Indeed, when the last reaction was carried out in the absence of cinnamic acid (Ph3P, DEAD, refluxing toluene, 1.5 h), the oxetane 15 was isolated as a main reaction product in 71% yield. The assignment of stereochemistry at the C-5 in product 15 was confirmed by an NOE interaction between H-1 and H-5, indicating that these protons are in close proximity on the same side of the ring. Compound 15 might be of use for a synthesis of a hitherto unknown conformationally constrained analogue of 1 (5,7-anhydrogoniofufurone). Hydrolytic removal of the cyclohexylidene protective group in 14 gave the expected lactol 16 (59%), which upon treatment with Meldrum’s acid (12) Mitsunobu, O. Synthesis 1981, 1. (13) Mata, F. Z.; Martinez, M. B.; Perez, J. A. G. Carbohydr. Res. 1990, 201, 223. (14) The side product 11 was presumably formed as a result of the competitive intramolecular dehydration of 10 (in furanose form), during removal of acetic acid by codistillation with toluene. (15) Yang, M.; Li, H. M.; Zhao, G.; Yu, Q.-S.; Ding, Y. Chin. J. Chem. 2000, 18, 225.
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Scheme 3.
Synthesis of (+)-Crassalactone C (3)
in the presence of triethylamine gave (+)-crassalactone C (3), with physical and spectral properties in reasonable agreement with those reported in the literature.8 Compounds 1-3 were evaluated for their in vitro antiproliferative activity toward human myelogenous leukemia (K562), Burkitt’s lymphoma (Raji cells), cervix carcinoma (HeLa), and normal fetal lung (MRC-5) cell lines. Cytotoxic activities were evaluated by using standard MTT assay16 after exposure of cells to the tested compounds for 72 h. The commercial cytotoxic agent doxorubicin (DOX) was used as a positive control in this bioassay. The results are shown in Table 1.
Table 1. In Vitro Cytotoxicity of 1-3 and DOX IC50, µMa compd
K562
Raji
HeLa
MRC-5
1 2 3 DOX
0.41 0.028 3.56 0.25
18.45 1.25 15.46 2.98
8.32 0.89 11.25 0.07
>100 >100 >100 0.10
a IC is the concentration of compound required to inhibit the cell growth 50 by 50% compared to an untreated control.
In general, all three natural products 1-3 exhibited antiproliferative activities against all of the malignant cell (16) Scudiero, D. A.; Shoemaker, R. H.; Paull, K. D.; Monks, A.; Tierney, S.; Nofziger, T. H.; Currens, M. J.; Seniff, D.; Boyd, M. R. Cancer Res. 1988, 48, 4827. 4237
lines (K562, Raji, and HeLa) but were devoid of any cytotoxicity against the normal fetal lung fibroblasts (MRC5). 7-epi-(+)-Goniofufurone (2) was found to be the most efficient cytotoxic agent against all neoplastic cells, with IC50 values ranging from 0.028 to 1.25 µM. The most pronounced antiproliferative activity of compound 2 was recorded against the K562 cells, being ca. 9-fold more potent than the commercial cytotoxic agent doxorubicin. Against the Raji cells, 7-epi-(+)-goniofufurone (2) was over 2-fold more active with respect to the reference compound, doxorubicin. This compound was also active against HeLa cells, but it was almost 13-fold less potent than the reference compound (DOX). Remarkably, compound 2 exhibited 1 order of magnitude higher cytoxicity against all the malignant cell lines when compared to (+)-goniofufurone (1) and (+)crassalactone C (3). The previous biological data2 in different neoplastic cell lines suggested that (+)-goniofufurone (1) was more active than 7-epi-(+)-goniofufurone (2). It appears that K562, Raji, and HeLa represent the first neoplastic cell lines in which stereoisomer 2 shows a stronger in vitro antiproliferative activity with respect to (+)-goniofufurone (1). In conclusion, by utilizing a chiral pool strategy based on D-xylose as the starting material, we have completed a new divergent synthesis of the natural styryl lactones (+)goniofufurone (1) and (+)-7-epi-goniofufurone (2), as well as the first total synthesis of (+)-crassalactone C (3), a novel 7-O-cinnamoyl (+)-goniofufurone derivative that has been recently isolated from the tropical plant Polyalthia crassa.8 (17) Bols, M. Carbohydrate Building Blocks; John Wiley & Sons Ltd.: New York, 1996.
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In addition to providing a divergent access to the natural products 1-3, this approach is flexible and straightforward. It uses nonexpensive reagents and a readily available starting material.17 These advantages make the synthetic methodology suitable for easy preparation of a variety of 7-O-substituted (+)-goniofufurone analogues for biological evaluation. In a preliminary MTT bioassay, all three natural products 1, 2, and 3 showed remarkable in vitro antiproliferative activities against K562, Raji, and HeLa neoplastic cell lines. 7-epi(+)-Goniofufurone (2) was found to be the most active compound, being at least 1 order of magnitude more cytotoxic than (+)-goniofufurone (1). To the best of our knowledge, the K562, Raji, and HeLa cells represent the first malignant cell lines, for the time being, in which 7-epi-(+)goniofufurone (2) exhibits more potent in vitro antiproliferative activity with respect to (+)-goniofufurone (1). Finally, based upon observed antitumor activities of 1-3, as well as their inactivity against the normal MRC-5 cells, we believe that these natural products may serve as important leads in the synthesis of more potent and selective antitumor agents derived from the parent compounds 1-3. Acknowledgment. Financial support from the Ministry of Science of the Republic of Serbia (Project No. 142005) is gratefully acknowledged. Supporting Information Available: Experimental procedures and full spectroscopic data for all compounds. This material is available free of charge via the Internet at http://pubs.acs.org. OL701734S
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